An electrical power conversion circuit is a circuit in which the form of the electrical power is changed to adapt a power source having predetermined voltage and current to a load requiring a different predetermined voltage and current, it being assumed that the power available from the source is adequate to meet the power demand of the load. The power conversion circuits to which the present invention relates, may be further characterized as reasonably efficient, and not effecting the necessary change in the form of the electrical power through dissipation.
The usual desired conversions are adapting a power supply having a higher output voltage/current to a load requiring a lower supply voltage/current or adapting a power supply having a lower output voltage/current to a load requiring a higher supply voltage/current. Assuming high conversion efficiency, making the power output substantially equal to the power input, the voltage-ampere products (assuming a resistive load) will be substantially the same at the source and at the load. Assuming a fixed VA product at the input of a converter, reducing the voltage at a resistance load is accompanied by an increase in load current and increasing the load voltage is accompanied by a decrease in load current.
A further necessary feature of an electrical power conversion circuit is that it be able to accommodate bidirectional energy flows which are present when the loads are reactive and must periodically return stored energy to the source. Reactive loads are, of course, commonplace.
A transformer is by far the most commonly used electrical power conversion device but it is restricted to AC electrical power conversion. Transformers are efficient in transforming AC voltages and currents supplied to their primary windings to suit the requirements of loads connected to their secondary windings. Transformers also accommodate bidirectional energy flows (with the complementary voltage and current conversion factors noted above).
Electrical transformers, though simple in construction, reliable in operation and quite efficient, are not without disadvantages. As power levels increase, they become quite bulky in size and heavy in weight, a distinct disadvantage in many special applications, such as avionics. Since power transformation is set by the turns ratio, the conversion factors of a power transformer can not be varied without some sort of tap-changing mechanism. While the efficiencies of modern power transformers may be quite high, they nevertheless do impose some losses, which may become significant. Moreover, under certain conditions, power transformers will introduce waveform distortions in their outputs, which in certain applications cannot be tolerated. Consequently, additional wave-shaping or filtering networks must be utilized.
The final and particularly significant drawback of power transformers is that they are strictly AC devices, and thus cannot transform DC or very low frequency AC input power.
A proposed solution for bidirectional AC/DC electrical power conversion has been described in a patent. In the described circuit, the ungrounded terminal of the power source is connected to one terminal of a first "series" switch, the other switch terminal being connected at a node to one terminal of an energy storing inductor, the other inductor terminal being connected to the ungrounded terminal of the load. The grounded terminal of the power source is connected to the grounded terminal of the load and a second, shunt connected, switch is provided connected between the node and ground. An energy storing capacitor is connected between the two load terminals.
The switches of the described circuit, which are characterized as "bidirectional chopper switches", are operated cyclically with one switch being maintained in an alternate state to that of the other switch. The duty cycle is then adjusted to adjust the voltage conversion (or current conversion) ratio between source and load. The suggested switching device is a bipolar junction transistor installed across the DC terminals of a four diode bridge to facilitate bidirectional operation, the AC terminals of the bridge providing the switching terminals.
In the described circuit, control of the switching is achieved by a direct connection from a single pwm oscillator to the base of one switching transistor, and an indirect connection to the base of the other switching transistor via an inverter. Since the emitters of both transistors are returned to one of the DC terminals of the bridge, one emitter of which may be at the source (or load) potential, and the other emitter of which may be at ground potential, it is apparent that an isolated drive to the input junctions, not referenced to the circuit potentials or to the drive supplied to the other switch may be necessary to a practical form of the described circuit.
While sound in principle, the described circuit assumes ideal switching devices but the described devices are less than ideal and the circuits themselves place severe demands upon the switching devices in delivering significant power to a load.
Bipolar junction transistors, particularly power transistors, are known to store significant amounts of charge in the junction region. Should a control voltage be applied to turn one transistor off as a complimentary control voltage is being applied to turn the other transistor on, the flow of current in the first transistor would continue for sometime after the turn off control had been applied, and simultaneous conduction in both transistors would result. Simultaneous conduction by the two transistors causes a short circuit across the power source and may be expected to produce potentially damaging currents.
Unfortunately, a simultaneous off condition for both switches is little better. Should the switching be so timed that the first transistor is turned off before the second transistor is turned on, the series inductor in circuit with the opening switch would seek out a path to discharge the energy stored in its magnetic field. In discharging through the opening switch, that switch is stressed and likely to product potentially damaging voltages.
Further complicating the picture in using paired bipolar junction power transistors is the requirement of significant current for the base drives which further complicates the achievement of synchronized commutation beyond that of a more purely voltage controlled device.
In realizing a practical bidirectional AC/DC electrical power conversion circuit similar to the described circuit, the present invention addresses the need for selection of more practical semiconductor switches and for a commutation circuit which avoids placing unnecessary commutating stresses upon the switches.